As the duct hose mentioned above, there have been popularly used hoses which comprise a hose wall formed by spirally winding a tape of a soft polyvinyl chloride resin (a plasticized polyvinyl chloride resin) and bonding its side edges each other, and a hard spiral reinforcement comprising a hard wire covered with a polyvinyl chloride resin or a hard spiral reinforcement of a hard polyvinyl chloride resin (non-plasticized polyvinyl chloride resin).
Usually, a hose wall of a duct hose of a synthetic resin is formed in the form of a bellows structure to give a flexibility, and the hose is transported or stored in a compressed state for compactness. As to storing circumstances, a storing temperature is in a wide range of as low as around -20.degree. C. to as high as around 70.degree. C., and it is often stored for quite a long period.
In the above case, when trying to lay a duct hose formed mainly with a vinyl chloride resin, the duct hose can not be reverted to the original length in a normal state because of its having been compressed for storage and caused shrinking kinks. As a result, the length of the hose is much shorter than the original length and consequently the hose is required to have the additional length allowing for shrinkage. In addition to such problems, there has been a problem that inwardly folded wall due to shrinkage results in an increased pressure loss.
For solving the problems of a vinyl chloride resin such as the formation of the shrinking kinks, there has been used mainly, as a synthetic resin material having a high impact resilience and hardly shrinks, a thermoplastic polyurethane resin, particularly a polyester-series thermoplastic polyurethane resin which is inexpensive, to form a duct hose.
However, a polyester-series thermoplastic polyurethane resin is degraded by ultraviolet rays, and the ester bonds of the polyester-series thermoplastic polyurethane are cleft with hydrolysis by exposure to a high temperature and high humidity for a long time, e.g., rain or the conveyance of aqueous liquids and the like. As a result, its physical properties such as wear resistance, flexibility and resilience are deteriorated.